Use alternate any_multiple_needles detection

[DavisVaughan]

Closes tidyverse/dplyr#6835

We can't use the simple loc < size_needles check to see if we are in the "extra" match section. When a filter is involved, that isn't enough because we could have filtered out a match that occurred before the extra matches, so the extra match could actually be the first one.

However, since we process the needles in sequential order (i.e. needles 1 containment group 1, needle 1 containment group 2, needle 2 containment group 1, etc) we can track the previous needles loc in loc_needles_previous as a more correct way to see if we've already seen 1 match for that particular needle.